The invention relates to a display tube comprising an electron gun in an evacuated envelope for generating an electron beam which is focussed on a display screen with the aid of an electrostatic focussing lens and which is deflected across the display screen in two mutually perpendicular directions.
A display tube of this type is used, for example, in a device for displaying symbols and/or characters generated by, for example, a computer. Such a tube is also referred to as a D.G.D. tube (DGD=Data Graphic Display). Such a display tube may, however, alternatively be a projection television display tube or another type of display tube in which only one electron beam is generated.
A display tube of this type is known from "Philips Data Handbook", Electron tubes, part 8, July 1983, Monitor Tubes.
An electron beam spot of very high quality is desired on the display screens of both projection television display tubes and DGD tubes. This is a spot having very determined, preferably small dimensions and without a halo surrounding the spot. For example, the spot must be circular. In the gun types known hitherto having focussing lens electrodes deep-drawn from sheet material it has been difficult to realize the desired spot circularity.
Further, an asymmetrical halo (haze) may occur around the core of the spot when the triode grids of the electron gun are not exactly in alignment. This asymmetry of the core halo results in an enlargement of the spot upon focussing to the starting point of the halo (the situation in which the halo has completely disappeared).